1. Field of the Invention
The present invention relates to a crane, which is capable of loading and unloading cargoes, such as box-type containers, at harbors, for example, and a method for controlling the crane. More specifically, the present invention relates to a crane and a method for controlling the crane which enables landing of a cargo at a predetermined position with high accuracy in a short period of time.
2. Description of Related Art
The operations of loading containers from a trailer to a ship or unloading of containers from a ship to a trailer is carried out, for example, in harbor yards using cranes.
FIG. 10 is a diagram showing a crane which may be used for the loading and unloading operations.
As shown in FIG. 10, the crane 1 is a bridge crane, which is called a container transfer crane (hereinafter referred to as a “crane”) capable of loading a container Ca, which is hoisted by the crane, into a target container Cb.
The crane 1 includes a crane traveling body 2, upper bars 3, a traverse trolley 4, a hanging member 5, rope members 6, and a hoisting device 7. In this crane 1, the traverse trolley 4 moves in the horizontal direction along the upper bars 3 of the crane traveling body 2, and the hanging member 5 called a spreader, which supports the cargo, is hung from the traverse trolley 4 by the rope members 6 so that the hanging member 5 can be raised and lowered by winding and unwinding the rope members 6 using the hoisting device 7 which is disposed at an appropriate position on the traverse trolley 4 or the crane traveling body 2. Also, the cargo may be moved in parallel by moving the transverse trolley 4 along the upper bars 3 of the crane traveling body 2.
When the container Ca, which is the cargo hoisted by the crane 1, is placed on a predetermined target container Cb and stowed, it is necessary to prevent any displacements in the horizontal position between the hoisted container Ca and the target container Cb exceeding an allowable value.
Also, prior to hoisting the container Ca, it is necessary to accurately land the hanging member 5 on the container Ca so as to prevent any horizontal displacements in position exceeding the allowable range. It requires great skill in this type of operation to land the hanging member 5 onto this kind of container Ca within the allowable range of horizontal displacement. It is also a very time consuming operation.
Accordingly, various proposals have been made for techniques to control the stowage of the container, whose main function is the automation of landing operations, and they are disclosed in, for instance, Japanese Unexamined Patent Application, First Publication No. Hei 10-120362, and Japanese Patent No. 2,813,510.
Japanese Unexamined Patent Application, First Publication No. Hei 10-120362 discloses a landing control technique in which the degree of oscillation of the container Ca, which is hung from a crane, from moment to moment is measured using a detector, and the horizontal position of the container Ca is estimated by an operation using the oscillation rate of the hoisted container Ca, which is computed based on the change in the oscillation over time. Also, factors, such as the position, and speed of a transverse trolley, are controlled if necessary. The speed of descent of the hoisted container Ca is adjusted so that the container Ca, the position of which is computed as described above, lands on the predetermined position of the target container Cb with suitable timing so that the shift in position in the horizontal direction falls within the allowable range at the moment the container Ca lands on the target container Cb.
Also, the gist of the landing control technique disclosed in the above-mentioned Japanese Unexamined Patent Application, First Publication No. Hei 10-120362 is to estimate the position of the cargo hung from the crane using a model indicating the dynamic behavior of the hoisted container Ca and the rope members 6.
However, the dynamic model cannot cover all the factors affecting the estimation of the position of the container Ca hung from the crane. In particular, there is a possibility that an error in the estimation of the horizontal positional displacement may be caused due to difficulty in modeling the effect of disturbance. Here, examples of disturbances having a large effect include wind, the weight distribution of the cargo in the container Ca, and unbalanced tension of the rope members 6. The horizontal positional displacement of the hoisted container Ca at landing may exceed the allowable range if such effects are too large.
On the other hand, U.S. Pat. No. 2,813,510 discloses a technique in which a mechanical guide is extended from the bottom of the container Ca so that the container Ca may be positioned on the container Cb along the guide. Although this technique functions to correct the above-mentioned problem of horizontal positional displacement, the weight to be lifted by the hoisting device 7 is increased since the mechanical guide is an attachment to the hanging member 5, and hence, the driving capacity of the hoisting device 7 needs to be increased. Also, mechanical contact of the guide with the target container is inevitable, and therefore, there is the problem that the mechanical guide and the container Cb tend to be easily damaged.
Also, problems in landing errors due to errors in estimating the position of the container Ca hoisted by the crane can be solved, if the degree of positional displacement measured at that time is within the allowable range of landing accuracy, by landing the container Ca before the positional displacement exceeds the allowable range of landing error.
That is, there will be no problem if the time required for landing is shorter than the time over which the degree of positional displacement will exceed the allowable range, by immediately starting the descent of the hoisted container Ca, when the horizontal positional displacement measured at that time is within the allowable range.
However, there is a restriction on the speed of descent of cargo from the viewpoint of safety, to decrease the impact upon landing, and hence, it is necessary for the vertical distance between the container Ca hoisted by the crane and the target container Cb be sufficiently small in order to land the container Ca before the positional displacement exceeds the allowable range.
As an example, assume that the container Ca is hoisted by using the rope member 6, the length of which is 10 meters from the top to the bottom, and the container Ca is lowered to land by winding down the rest of the rope member. Also, assume that the allowable range of horizontal positional displacement is 30 mm. In this state, the cycle of the rope member 6 is about 6.3 seconds (2π√(10/9.8). Moreover, assuming that the container Ca is oscillating in the moving direction of the traverse trolley 4 at a half amplitude of 100 mm, the average speed of the container Ca in the horizontal direction is about 63 mm/sec.
Accordingly, if the lowering of the container Ca is started at the moment that the positional displacement between the container Ca and the target container Cb is detected to be zero by a horizontal position displacement detection means, the time for lowering the container Ca needs to be about 0.48 sec or less in order to satisfy the allowable range (30 mm or less) at the landing. That is,Time for lowering=30 mm/63 mm per sec=0.476 sec
Here, if the average speed for lowering the container Ca is restricted to 100 mm per sec, the distance between the container Ca and the target container Cb in the height direction needs to be 48 mm or less (i.e., 100 mm/sec×0.48 sec=48 mm).
Prior to landing, if the positional displacement does not fall within the allowable range of landing accuracy, it is necessary to correct the positional displacement or wait for the positional displacement to fall within the allowable range. However, if a correction is made for the positional displacement or if waiting for the positional displacement to fall within a desired range, it is necessary to prevent interference with the movement of the hoisted container Ca by the contact with the target container Cb during that period.
That is, it is necessary that there be vertical space between the container Ca and the target container Cb, and this space must be maintained at the above-mentioned value or less.
In order to maintain the above-mentioned space, it is a prerequisite that the distance between the two containers measurable. There are various methods for measuring the distance between the container Ca and the upper surface of the target container Cb, however, all of them have problems in measuring a distance on the order of the above-mentioned value.
For instance, there is a method in which the position of the hoisted container Ca is detected based on the length of the rope or by using an electro-optical distance meter to obtain the difference between the two provided that the height of the upper surface of the target container Cb is given. However, in practice, errors in the height of the stowage location of the target container Cb, errors in the height of the container, errors caused by stretching of the rope members 6, errors due to structural deformation of the crane 1, etc., accumulate, and it is difficult to carry out a measurement which is satisfactory for the above purpose.